This invention relates to air brakes for motor vehicles, and more particularly it is concerned with an antiskid device for air brakes which enables to apply the brake to stop the vehicle in a short braking distance without causing the hazard of skidding when the brake is applied.
The slip rate which indicates the degree of skidding of a wheel rotating on the road surface while the brake is being applied can be expressed as follows: ##EQU1## WHERE Vv is the speed of the motor vehicle and Vw is the speed of the wheel. When S = 1, the rotational movement of the wheel is stopped by the high braking force in spite of the fact that the motor vehicle is still in operation, so that the wheel is brought to a locked condition. This presents a hazard because the motor vehicle tends to slip sideways. When S = 0, the speed of the motor vehicle is the same as that of the wheel and no braking force is transmitted from the wheel to the road surface, so that it is impossible to stop the vehicle unless air resistance or the gradient of the road is utilized. As is well known to a person skilled in the art, S = 0.2 or thereabout is a slip rate which is most desirable because the hazard of sideways skidding can be avoided and the vehicle has a short braking distance.
Antiskid devices for air brakes are designed, based on the aforesaid theory, to maintain the slip rate at about 0.2 when the brake is applied. An antiskid device of the prior art comprises a modulator mounted in a pressurized air line connecting a brake valve to brake actuators, and a signal producing means for producing a pressure reducing signal when the braking pressure applied by the brake actuators has reached a pressure increase target value, the pressure reducing electric signal actuating the modulator to reduce the braking pressure applied by the brake actuators, whereby the slip rate of the wheels can be maintained at an optimum level of 0.2 or thereabout.
Some disadvantages are associated with antiskid devices of the prior art constructed as aforesaid. For example, there is a time lag of the initiation of a reduction in the braking pressure caused by the operation of the modulator behind the production of a pressure reducing electric signal. This has hitherto caused a rise in the braking pressure applied by the brake actuators to an unnecessarily high level, with the result that skidding of a moderate degree has occurred or the braking force has not been high enough.